This invention relates to a gas chromatographic apparatus and method wherein a constant flow rate is maintained during analysis of a sample. In another aspect, the invention relates to a method which is effective in checking or redetermining the calibration of a gas chromatographic apparatus quickly and easily.
Gas chromatography instruments for the chemical analysis of samples typically include a sampling means for ejecting the sample in vaporized form therefrom, a gas chromatography column for separating components of the vaporized sample, and a detector which detects the components as they elute from the column. An inert carrier gas is employed to sweep the gaseous sample through the column. Such an apparatus can also conveniently include a recording device which records the outlet of the detector in a conveniently readable form, such as peaks on a chart.
Each peak corresponds to a particular component and has an amplitude which is a function of concentration where a concentration sensitive detector is used. In addition, an area, denoted simply as peak area, is defined under each peak which is representative of the amount (i.e., in grams) of its corresponding component in the sample. Stated another way, peak area is the integrated output of the detector for a particular detected component.
Ideally, peak area should vary solely in response to a change in the amount of the component detected. In actual practice, however, peak areas also change in response to changes in extraneous conditions such as temperature, barometric pressure and flow rate into the detector. Thus, any change in any one of these conditions during analysis can lead to erroneous results. It is, therefore, important that all extraneous conditions be maintained substantially constant to the extent possible.
Maintenance of a constant flow rate into the detector has been a particularly critical problem in prior gas chromatography systems. There are two primary causes for undesirable changes in flow rate. First, the flow rate of a component tends to vary according to the component concentration as the component elutes from the separation column. Second, a component in the column will tend to unpredictably affect the flow rate of another component which has already exited the column. This is known as the "slugging" effect.
Another important problem in gas chromatographic analysis is the nonlinear response of virtually all detectors. That is, the amplitude of the detector's output signal is not a linear function of the parameter, such as concentration, being detected. This also means that peak area is typically a nonlinear function of the amount of the corresponding component in the sample. Such nonlinearity dictates that calibration be carried out in order to obtain accurate results.
Prior calibration techniques have typically involved eluting a number of standard samples of known mass through the separation column, and then determining the peak area corresponding to each standard sample. A calibration curve is then plotted by plotting a point for each sample, wherein each point has a first coordinate corresponding to the ratio between a quantitative measurement (i.e., mgs of carbon) of the sample and peak area, and a second coordinate corresponding to peak area. A calibration factor can be derived for any peak area which can be used to determine the actual amount of a component associated with a particular peak.
The above described calibration technique is very slow and tedious to carry out since each standard sample must be individually weighed or volumetrically measured, and since each vaporized standard sample must then be passed through the separation column.